Activation of peripheral T cells, as well as fates of differentiating thymocytes during the processes of positive and negative selection, are determined by the relative strength of the T-cell receptor (TCR) signal generated during interactions with peptide/self major histocompatibility complex (MHC) antigens. The strength of this signal is dependent partly on activities on protein tyrosine kinases (PTKs) associated with the TCR. Immature thymocytes expressing TCRs with high affinities for self- peptide/MHC are thought to generate excessive PTK-mediated signals that result in clonal elimination. In contrast, thymocytes expressing lower avidity TCRs for self-peptide/MHC are thought to transduce signals that result in continued differentiation and selection of the peripheral TCR repertoire. Moreover, thymocytes are believed to be capable of responding to relatively weak TCR signals during selection that are below the threshold for activation of peripheral T cells. The increased signaling requirement for activation expressing self-reactive TCRs are demonstratable under normal circumstances. This diminished capability of TCR signals to activate peripheral T cells may be related to decreased activity of TCR-associated PTKs. However, any roles for PTKs in setting the T-cell activation threshold are not defined. The hypothesis of this proposal is not inappropriate peripheral TCR signaling may occur through excessive PTK expression, resulting in activation of autoreactive T cells and autoimmunity. Unlike Lck, Fyn and Zap-70, Syk is down-regulated during T-cell development. The importance of decreasing Syk expression during T cell development is not known, but one possibility is that it may serve to increase the activation threshold of nature T cells. Therefore, Syk may participate in the maintenance of peripheral tolerance and is the most likely candidate PTK to e involved in initiation of autoimmunity. The aims of this proposal are: 1) to determine whether up-regulation of Syk expression in mouse T-cell hybridomas can lead to enhanced TCR signaling and autoreactivity in vitro, and 2) to determine whether enhanced TCR signaling in the periphery can result in autoimmunity in vivo by developing transgenic mouse models in which Syk is overexpressed. T-cell hybridomas and transgenic mice will be generated that either constitutively express human Syk under control of the CD38 enhancer- promoter or inducible expressed human Syk under control of the reverse tetracycline-controlled transactivator system. The goal of this project is to understand the roles of TCR signaling and Syk in the pathogenesis of autoimmunity.